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AS1920 Datasheet PDF : 14 Pages
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AS1920/AS1922
Datasheet - Application Information
9 Application Information
Supply Monitoring
The AS1920/AS1922 are low-power, high-accuracy triple-supply monitoring circuits with a single 200ms microproces-
sor reset output.
All VCC inputs (VCC18, VCC3, and VCCA) must be above their thresholds to prevent a reset from being asserted. The
AS1920/AS1922 will assert a reset during powerup, powerdown and brownout conditions on one or all of the VCC
inputs.
Power Detect
The AS1920/AS1922 is powered from the 3V input pin (VCC3). The power detect circuitry ensures that the device pulls
pin RESETN low as soon as the primary input pin is ≥1V or the secondary input pin is ≥1V.
Powerup
The power detect circuitry ensures that RESETN will be low when either VCC18 or VCC3 ≥1V. As long as any one of the
VCC inputs is below its reset threshold, RESETN will stay at a logic low. When all VCC inputs have stabilized, an inter-
nal timer will be started and RESETN will be driven high after 200ms.
Powerdown
On powerdown, if any of the VCC inputs drop below its threshold, RESETN is held at a logic low (0.3V) and is guaran-
teed until VCC3 and VCC18 < 1V.
Override Functions
As the trip points for VCC3 and VCC18 will always be greater than the trip point for VCCA, if pin VCCA is not used, it
should be tied to VCC3 or VCC18. In a similar manner if pin VCC18 is not used it should be tied to VCC3. Tying VCC3 to
VCC18 and operating from a 1.8V supply will assure the continuous assertion of RESETN.
Note: Pin VCC3 must be used in all configurations.
Ensuring RESETN Valid for Supply Voltages < 1V
When the supplies drops below 1V the RESETN output current sink capability is greatly reduced. Stray currents and
stray capacitance to signals other than GND can cause the RESETN output to float. This is not relevant for most appli-
cations since most microprocessors and other circuits do not operate with a supply voltage < 1V.
In applications where the RESETN output must be valid down to 0V, a pulldown resistor from pin RESETN to ground
will ensure RESETN is held low (see Figure 12).
Figure 12. Typical Application w/Resistor on RESETN Output to GND
5
3V
VCC3
1.8V
4
VCC18
Adjustable
Supply
R1
3
VCCA
R2
AS1920/
AS1922
1
RESETN
2
GND
R3
100kΩ
The value of the pulldown resistor (R3) is a trade-off between pulldown strength and loading of the RESETN pin. If the
value of R3 is too large RESETN may still float, and if the value of R3 is too small it may load RESETN and also use
excess supply current, therefore a value of 100kΩ is recommended for the pulldown resistor.
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